Make selection highlight color configurable

A few comments on this commit:

  https://github.com/wincent/Command-T/commit/d30099cac79dfdbc4a16d

suggest that there is no "one size fits all" selection highlight color
that's going to work for everybody.

Make it configurable.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Bump CommandTMaxFiles default to 30,000 files

Computers are getting faster, so it seems reasonable to bump the
default.

Signed-off-by: Wincent Colaiuta <win@wincent.com>